Advances in thin film technology have been leveraged to create devices using microelectromechanical systems (MEMS) elements. MEMS elements are typically capable of motion or application of a force. Devices using MEMS elements have been developed for a wide variety of applications due to their low cost, high reliability and extremely small size. MEMS elements have been utilized as microsensors, microgears, micromotors and other microengineered components. One important application of such MEMS devices has been in free-space optical switches for fiber optic communications systems. MEMS optical elements, e.g., in the form of rotatable MEMS mirrors, are arranged in square or rectangular arrays called a switch fabric. The switch fabric is aligned with two or more corresponding arrays of optical fibers. The mirrors move into position in which they can selectively couple light from a fiber in one array to a fiber in another array.
In one type of prior art free-space optical switch; MEMS mirrors are attached to a substrate by flexures. The mirrors rotate under the influence of magnetic force from an “OFF” position substantially parallel to the substrate to an “ON” position substantially perpendicular to the substrate. In the “ON” position, the mirror intercepts an optical beam from a fiber in an input array and deflects the beam toward a fiber in an output array. A top chip attached to the substrate has openings that align with the MEMS mirrors. The openings in the top chip provide reference stopping planes for the MEMS mirrors so that they are properly aligned perpendicular to the substrate in the “ON” position. A voltage applied between a particular mirror and the top chip provides an electrostatic force that retains the mirror in the “ON” position.
When scaling to larger optical switch fabrics (e.g., 16×16, 32×32), the yield of the optical MEMS die will decrease with the increasing die size. This places a feasible upper bound on such scaling. One proposed solution to this problem is to develop a new technology with a finer pitch and, therefore, a smaller die. Unfortunately this is a lengthy development process. Another alternative solution is to use redundant mirrors on the device die. Unfortunately, this complicates the overall design of the optical switch.
It is known to tile two or more smaller dies together to form a larger device. For example, Minowa et al. uses four 4×4 arrays tiled together in a mosaic fashion to form an 8×8 array. However, for 16×16 arrays and larger, the size of the array still presents problems even if smaller devices are tiled together. For example, as the array size increases the distance between input and output fibers increases. The increased optical path between the fibers can lead to undesirable beam spreading. The beam spreading may be overcome by placing collimator lenses between the arrays. However, the alignment of the collimator lenses to the switching elements is difficult and even slight misalignment will result in optical loss that degrades switch performance. Another problem with tiling two or more dies is that the dies must be very accurately aligned with each other in order to ensure that the mirrors on one die will align with those on the other dies in the mosaic.
Prior art alignment techniques include self-alignment and active-alignment. In self-alignment, metallized bonding pads are placed on two different pieces, e.g. a MEMS device die containing rotating mirrors and a corresponding top chip. Solder is applied to the bonding pads and the two pieces are brought together such that corresponding bonding pads roughly align with each other. When solder is heated through reflow, surface tension forces between the solder and the bonding pads pull the two pieces into alignment. In active-alignment, the pieces are placed, within micron tolerances, using a pick and place tool and held in place until the solder freezes. Active-alignment allows for the use of epoxies as well as solders for attachment of the top chip to the device die.
However, even using these techniques, alignment can be particularly problematic with a tiled device having four 8×8 MEMS mirror arrays totaling 256 MEMS mirrors.
Thus, there is a need in the art, for a self aligned or actively aligned optical MEMS device and a method for making it.